of the eggs of insects and matters connected with it, I pass on to
their _substance_ or their external and internal composition, giving
at the same time some account of the embryo included in them. The
eggs of insects, like those of birds, consist in the first place
of an external coat or shell, varying greatly, as to substance, in
different genera. Most commonly, particularly in those which deposit
their eggs in moist situations, as in dung, earth, and the like, it
is a mere membrane, often thin and transparent, and showing, as in
spiders, all the changes that take place in the inclosed embryo,
as the formation of the head, trunk, and limbs[139]. This membrane
is sometimes so delicate as to yield to the slightest pressure,
and insufficient to protect the included fluids from too rapid
an evaporation, if the eggs be exposed to the full action of the
atmosphere. In most _Lepidoptera_, and several other tribes, this
integument is considerably stronger, in those moths whose eggs are
exposed throughout the winter, as _Lasiocampa Neustria_, &c., so
hard as not to yield easily to the knife. Even in these, however,
its substance is more analogous to horn or a stiff membrane than
to the shell of the eggs of birds. Nothing calcareous enters into
its composition, and it is not perceptibly acted upon by diluted
sulphuric acid. The eggs of birds are lined by a fine membrane; but
I have examined several of those of insects, and have been able to
discover nothing of the kind in them. I will not, however, affirm
that it does not exist, though the shell of the insect egg appears
more analogous to the membrane that lines that of the bird than to
the outside shell itself.
Within this integument is included a fluid, on the precise nature
of which, except that it is an aqueous whitish fluid, few or no
observations have been made, or indeed are practicable; but it is
reasonable to suppose, that like the white and yolk of the bird's
egg, it serves for the development of the organs of the germe of the
future insect.
But few observations are recorded that relate to the embryo included
in the egg. It is stated, that it is invested with an extremely fine
and delicate pellicle--supposed by some analogous to the _Chorion_
and _Amnios_ of the human fœtus, though others think the shell of the
egg to correspond with the _Chorion_, and the successive integuments
of the larva with the _Amnios_[140]. When the egg is first laid,
nothing indeed is to be seen in it but the fluid just mentioned;
the first change in this fluid is the appearance of the head of
the embryo, more particularly in _Coleoptera_, of two points, the
rudiments of the mandibles, and of those apertures into the tracheæ
which I have called spiracles[141]; the little animal we may suppose
then assumes its form and limbs. The embryo is usually so folded in
the egg that the head and tail meet[142], and the head, annuli, and
other parts of the larva are often visible through the shell[143].
Swammerdam even saw the pulsation of the great dorsal vessel through
the shell of the egg of _Oryctes nasicornis_.
Under this head I must notice another singular circumstance peculiar
I believe to the eggs of insects, that sometimes, though rarely, they
are covered with down or hair. Those of a singular little hemipterous
insect, of a genus I believe at present undescribed, the ravages
of which upon the larch have been before noticed[144], are covered
by a downy web, as is the case with the animal itself. De Geer has
described the eggs of a bug, not uncommon in this country (_Pentatoma
juniperina_ Latr.), which are reticulated with black veins, in which
very short bristles are planted[145]. I possess also a nest of brown
eggs, probably of a species of the same genus, found upon furze,
which appear to be covered with very short downy hairs. The top of
these is flat, and surrounded by a coronet of short bristles, each
surmounted by a small white ball, so as to wear the appearance of
a beautiful little _Mucor_. But hairy eggs are not confined to the
Hemiptera Order, for, according to Sepp, those of the figure-of-eight
moth (_Bombyx cæruleocephala_) are of this description[146].
iv. _Number._ The fertility of insects far exceeds that of birds,
and is surpassed only by that of fishes[147]. But the number of
eggs laid by different species, sometimes even of the same natural
family, is extremely various. Thus the pupiparous insects may be
regarded as producing only a single egg; _Musca Meridiana_ L., a
common fly, lays two[148], other flies six or eight; the flea twelve;
the burying beetle (_Necrophorus Vespillo_[149]) thirty; May-flies
(_Trichoptera_ K.) under a hundred; the silk-worm moth about 500;
the great goat-moth(_Cossus ligniperda_) 1,000; _Acarus americanus_
more than 1,000[150]; the tiger-moth (_Callimorpha Caja_) 1,600; some
Cocci 2,000, others 4,000; the female wasp at least 30,000[151];
the queen bee varies considerably in the number of eggs that she
produces in one season, in some cases it may amount to 40,000 or
50,000 or more[152]; a small hemipterous insect, resembling a little
moth (_Aleyrodes proletella_ Latr.) 200,000. But all these are left
far behind by one of the white ants (_Termes fatale_ F. _bellicosus_
Smeath.)--the female of this insect, as was before observed[153],
extruding from her enormous matrix not less than 60 eggs in a minute,
which gives 3,600 in an hour, 86,400 in a day, 2,419,200 in a lunar
month, and the enormous number of 211,449,600 in a year: probably she
does not always continue laying at this rate; but if the sum be set
as low as possible, it will exceed that produced by any other known
animal in the creation.
v. _Size._ The size of the eggs is in proportion to that of the
insect producing them, though in some instances small ones produce
larger eggs than those laid by bigger species. Thus the eggs of
many _Aptera_, as those of that singular mite _Uropoda vegetans_,
and of the bird-louse found in the golden pheasant, are nearly as
large, it is probable, as the parent insect; while those of the
ghost-moth (_Hepialus Humuli_) and many other _Lepidoptera_, &c.
are vastly smaller. This circumstance perhaps depends principally
on the number they produce: the majority of them, however, are
small. The largest egg known, if it be not rather an egg-case,
is that of a spectre insect (_Phasma dilatatum_), figured in the
Linnean Transactions[154], being five lines in length and three
in width, which probably approaches near the size of that of some
humming-birds. The largest egg of any British insect I ever saw was
that of the common black rove-beetle (_Staphylinus olens_) sent me by
Mr. Sheppard--this is a line and half long by a line in width. But
we do not often meet with insect-eggs exceeding a line in length. A
vast number are much smaller: those of Ephemeræ are more minute than
the smallest grains of sand[155], and some almost imperceptible, as
those of the subcutaneous moths, to the naked eye. Commonly the eggs
laid by one female are all of the same size; but in several tribes,
those containing the germe of the _female_ are larger than those that
are to give birth to a _male_. This appears to be the case with those
of the Rhinoceros beetle (_Oryctes nasicornis_[156]), and according
to Gould with those of ants[157]. As the female in a vast number
of instances is much bigger than the male, it is not improbable
that this law may hold very extensively. It is stated, however, by
Reaumur[158], that the reverse of this takes place in the eggs of the
hive-bee, those that are to produce males being larger than the rest.
Another peculiarity connected with the present head is the
augmentation in bulk which takes place, after exclusion, in the eggs
of the great tribe of saw-flies (_Tenthredo_ L.), the gall-flies
(_Cynips_ L.), the ants (_Formica_ L.) and the water-mites
(_Hydrachna_ Maïll. _Atax_ F.). Those of the two former, which are
usually deposited in the parenchymous substance of the leaves, or of
the young twigs, of various plants, imbibe nutriment in some unknown
manner, through their membranous skins, from the vegetable juices
which surround them[159], and when they have attained their full size
are nearly twice as large as when first laid. Except in the eggs of
fishes, whose volume in like manner is said to augment previously
to the extrusion of the young, there is nothing analogous to this
singular fact in any other of the oviparous tribes of animals, the
eggs of which have always attained their full size when they are laid.
It is to M. P. Huber that we are indebted for the knowledge of the
fact that the eggs of _ants_ grow after being laid, a circumstance
favoured probably by the moist situation in which the workers are
always careful to keep them. By an accurate admeasurement he found
that those nearly ready to be hatched were almost twice as big
as those just laid[160]. A similar observation was made on the
red eggs of a water-mite (_Hydrachna abstergens_) by Rösel, who
conjectured that they draw their means of increase from the body of
the water-scorpions (_Nepæ_), of which they form so singular an
appendage[161], which opinion is confirmed by De Geer, who observes
that when the water-scorpions are covered by an unusual number of the
eggs of the water-mites, they grow weak and languid, and endeavour
to rid themselves of their parasitic appendages[162]. It is most
probable that the mite lately named (_Uropoda vegetans_), which is
often found planted as it were upon the bodies of various beetles, by
means of a long pedicle, through which, as the fœtus by an umbilical
chord and placenta, it derives its nutriment from the above animals,
is at first so fixed in the egg state, though before it is disengaged
from the pedicle it is hatched, since it is often found with its legs
displayed and quite active--this is the more probable, as the eggs of
the water-mite are fixed by a pedicle to the animals to which they
are attached[163]. I have met with a remarkable instance, in which
pedunculated eggs seem to draw nutriment from the _mother_, which
brings the pedicle still near to the nature of the umbilical chord.
Those of the small hemipterous insect which infests the larch before
alluded to, are attached to the anal end of the mother by a short
foot-stalk not longer than the egg.
Dr. Derham seems to have observed, that the eggs of some _Diptera_,
of the tribe of _Tipulidæ_, also increase in size before the larva
is excluded[164]. It seems to me likely enough, that in this and
many of the above cases in which the egg is supposed to grow, it is
rather an extension of the flexile membrane that forms their exterior
proportioned to the growth of the included embryo from food it finds
within the egg, than from any absorption from without.
vi. _Shape._ We are accustomed to see the eggs of different species
of oviparous animals so nearly resembling each other in form, that
the very term _egg-shaped_ has been appropriated to a particular
figure. Amongst those of birds, with which we are most familiar,
the sole variations are shades of difference between a globular and
oval or ovate figure. The eggs of insects, however, are confined by
no such limited model. They differ often as much, both as to their
shape, sculpture, and appendages, as one seed does from another; and
it is not improbable that, if duly studied, they would furnish as
good indications of generic distinctions as Gærtner has discovered in
those of plants. Their most usual form indeed is globular, oval, or
oblong, with various intermediate modifications. We meet with them
ovate, or of the shape of the common hen's egg, flat and orbicular,
elliptical, conical, cylindrical, hemispherical, lenticular,
pyramidal, square, turban-shaped, pear-shaped, melon-shaped,
boat-shaped, of the shape of an ale-stand, of a drum, &c.[165], and
sometimes of shapes so strange and peculiar, that we can scarcely
credit their claim to the name of eggs. Thus the eggs of the gnat
are oblong and narrow, or nearly cylindrical, having at the top a
cylindrical knob[166], so as to give them the precise form of the
round-bottomed phial sometimes used by chemists: those of the common
water-scorpion (_Nepa cinerea_) are oblong, and at the upper end are
surrounded by a sort of coronet, consisting of seven slender rays or
bristles of the length of the egg[167], so as to resemble somewhat
the seeds of _Carduus benedictus_ (_Cnicus acarna_[168]) of the
old botanists. One would think this spinous circlet a very awkward
appendage to bodies which are to be gradually extruded through the
fine membranous ovaries and oviduct which inclose them: but they are
so admirably packed, the unarmed end of each egg fitting closely
into the space inclosed by the spines of the one next below it,
or, rather, the spines which are moveable, embracing it closely,
that not only is no room lost, but the ovaries are perfectly secure
from injury. The eggs of another species of this tribe (_Ranatra
linearis_) have only two of these spines or bristles--they are
inserted in the stem of a water-rush (_Scirpus_) or other aquatic
plant, so as to be quite concealed, and are only to be detected
by the two bristles which stand out from it[169]. The eggs of the
beautiful lace-winged flies (_Hemerobius_), those golden-eyed insects
so serviceable in destroying the plant-lice (_Aphides_[170]), are
still more singular. Those of _H. Perla_ are oval, and each of them
attached to a filiform pedicle not thicker than a hair, and seven or
eight times as long as the egg. By this pedicle (which is supposed
to be formed by a glutinous matter attached to one end, which the
female draws out by abstracting her ovipositor with the egg partly
in it from the leaf, to which she has previously applied it, to a
proper length, when the gluten becoming sufficiently solid she wholly
quits the egg,) the eggs are planted in groups of ten or twelve on
the surface of leaves and twigs, from which they project like so many
small fungi, to some of which they have a remarkable resemblance.
When the included larva has made its way out of them by forcing open
the top, they look like little vases, and were actually once figured
by a Naturalist, as we learn from Reaumur, as singular parasitic
flowers growing upon the leaves of the elder, for the origin of which
he was extremely puzzled to account[171]. Eggs similarly furnished
with a pedicle are also laid by other insects; but as most of these
have been before alluded to, it is not necessary to describe them
here[172]. The cause of these differences of form is for the most
part concealed from us: in many instances it may perhaps be referred
to that will to vary forms, and so to glorify his wisdom[173] and
power, independently of other considerations, which, as Dr. Paley
has well remarked[174], seems often to have guided the Great Author
of Nature. But in some cases the end to be answered is sufficiently
evident. The long footstalks of the eggs of the _Hemerobius_ just
mentioned, there can be little doubt, are meant to place them out
of the reach of the hosts of predaceous insects which roam around
them, from whose jaws, thus elevated on their slender shaft, they are
as safe as the eggs of the tailor bird in its twig-suspended nest
from the attack of snakes. Reaumur has described the eggs of a kind
of fly, common upon the excrements of the horse and other animals
(_Scatophaga vulgaris_ Latr.), or one related to it, that requires
to be immersed in the dung to which it is committed, on which the
future grubs are to feed. He found that if not thus surrounded with
moisture, they infallibly shrivelled up and came to nothing; but it
is equally necessary that they should not be wholly covered: if they
were, the young larva would be suffocated at its first exit from the
egg. In what way is this nice point secured? In this manner. Each
egg is provided at its upper end, at which the animal when hatched
comes out, with two diverging horns[175]; these prevent it from being
stuck into the excrement, in which the female deposits the eggs one
by one, more than three-fourths of its length: and when examined they
resemble not badly, as Reaumur remarks (except that their colour is
white), a parcel of cloves stuck into a pudding, as they are neatly
inserted at due distances in the disgusting mass[176]. The French
Naturalists found these eggs in swine's dung; I have observed them
in cow-dung. Latreille thinks that the bristles above described
attached to the eggs of _Nepa_ and _Ranatra_ have a similar use,
as the female plunges them all but these bristles into the stems
of aquatic plants[177]: but may not this have something to do with
their oxygenation? Reaumur has figured another egg of a dipterous
insect which has a longitudinal wing or lateral margin attached to
it, giving it the form of an oblong square, the object of which,
he conceives, is to give a greater surface by which it may be more
firmly fixed to the substance against which the fly attaches it[178].
Besides these more striking variations in figure, their surface,
though often smooth, is frequently curiously and most elegantly
_sculptured_, a circumstance that distinguishes the eggs of no other
oviparous animals. Some, as the margined egg just mentioned, are only
sculptured on one side, the other being plain; or, as those of the
Tusseh silk-worm[179] (_Attacus Paphia_) and other _Bombyces_, which
have orbicular depressed eggs with a central cavity above and below,
have their circumference crossed with wrinkles corresponding with
the rings of the inclosed embryo[180]. Others again are sculptured
all over. Of these, in some, the sculpture of the two sides is not
symmetrical, as in those of a fly figured by Reaumur[181]: but
in general there is a correspondence in this respect between the
different parts of the egg. In those elegant ones before alluded
to of some bird-louse attached to the golden pheasant, the shell
resembles the purest wax, and is scored with longitudinal striæ,
each distinguished by a series of impressed points, which give it
a beautiful appearance of net-work. In the others, as in a common
butterfly (_Hipparchia Ægeria_) and moth (_Geometra cratægata_),
the whole surface is covered with hexagonal reticulations[182].
Others, as those of another butterfly (_Hipparchia Hyperanthus_),
are beset with minute granules or tubercles[183]. Others again, like
those of the cabbage and hawthorn butterflies (_Pieris Brassicæ_ and
_Cratægi_), are remarkable for beautiful longitudinal ribs, often
connected by elevated lines crossing them at right angles[184]; and
in some, as in another butterfly (_Hipparchia Jurtina_), crowned by
imbricated scales[185]. Many other minor differences in this respect
might be noticed, but these will suffice to give some idea of the
infinite variety exhibited in this respect by these little atoms. If
the Creator has wrought them with so much art and skill, can it be
beneath his reasonable creatures to examine and admire them, that
they may glorify those attributes which they serve to illustrate?
Some eggs after exclusion occasionally become slightly corrugated:
Malpighi supposed that this occurs only when the eggs are barren,
having observed that those of the moth of the silk-worm which preserved
their plumpness always produced caterpillars, while those which
lost their original rotundity and became wrinkled were constantly
unprolific. Bonnet, however, found exactly the reverse take place
in another moth[186], so that these appearances are scarcely to be
depended upon. Kuhn asserts, that a virgin female of the puss-moth
(_Cerura Vinula_) having begun to lay eggs, which were yellow above,
green below, and depressed, he introduced to her an hour afterwards a
male, and some minutes subsequently to the union, she again deposited
eggs, which were wholly of a dark brown and convex[187].
vii. _Colour_. The colour of the eggs of insects is as various as
their shape and sculpture. They are very often white, those of some
spiders like minute pearls[188]; some are yellow, as those of the
silk-worm; others orange, such are the eggs of the bloody-nosed
beetle (_Timarcha tenebricosa_); others again of a golden hue;
sometimes they are of a sanguine red. I remember once being much
surprised at seeing the water at one end of a canal in my garden as
red as blood: upon examining it further I found it discoloured by
an infinite number of minute red eggs, belonging probably to some
dipterous insect of the Tipulidan tribe. There are also eggs of every
intermediate shade between red and black; some again are blue and
others green. They are not always of whole colours, for some are
speckled like those of many birds, of which I can show you specimens,
that are also shaped like birds' eggs; these I think were laid by a
common moth (_Odenesis potatoria_); others are banded with different
colours--thus the blue eggs of the lappet-moth (_Gastropacha
quercifolia_) are encircled by three brown zones[189]; others are
brown with a white zone[190].
Many eggs assume a very different colour after being laid a few
days. In general upon their first exclusion they are white. Those
of the chameleon-fly (_Stratyomis Chamæleon_) which I once found in
great numbers, arranged like tiles on a roof one laid partly over
another, on the under side of the leaves of the water-plantain,
from white become green, and then change to olive green. Those of
the hemipterous enemy of the larch, more than once mentioned in
this letter, are first mouse-coloured, then they assume a reddish
hue, and lastly a blackish one. Those of the gnat from white in a
short time assume a shade of green, in a few hours they are entirely
green, and at length become gray[191]. Those of the silk-worm,
which at first are of a yellow or sulphur colour, acquire a violet
shade. The eggs of that rare moth _Endromis versicolor_, are at
first sulphur-coloured, then green, next rose-coloured, and lastly
blackish. The colour of almost all eggs changes when they are near
hatching; but this change depends more frequently upon the colour of
the included larva, which appears through the transparent shell of
the egg, than upon any actual alteration in the egg itself.
viii. _Period of hatching._ The general rule for the hatching
of the eggs of insects is the absorption by the embryo of all
the superabundant moisture included in them; but the time varies
according to the state of the atmosphere, to the action of which they
are subjected. Like those of other animals, they require a certain
degree of heat for the due evolution of the included larva. This
heat in much the greater number of instances is derived from the
temperature of the air, but often also from other sources. The eggs
of the gad-fly tribe are hatched principally by the heat of the body
of the animal to which they are committed; and doubtless the vital
heat of various larvæ, small as it may be, must contribute something
to the hatching of the eggs deposited in them by various Ichneumons.
In the fermenting bark in which the instinct of the rhinoceros
beetles (_Oryctes nasicornis_ &c.) impels them to place theirs, the
dung which the _Scarabæidæ_ select for that purpose, and the decaying
vegetables chosen by many other insects, a degree of artificial heat
must exist: and the eggs, or rather egg-like pupæ, of the spider-fly
of the swallow (_Ornithomyia Hirundinis_) are hatched by the heat of
those birds which sit upon them along with their own eggs.
Fabricius says, "Insects never sit upon their eggs[192];" but
certainly, as I formerly related to you[193], the female earwig does
this, and one would be induced to suppose, from the circumstance of
the young ones following their mother, as chickens do the hen, that
_Pentatoma grisea_ (_Cimex_ Linn.), formerly mentioned, may do the
same[194].
With these exceptions, the eggs of all insects are hatched by
atmospheric heat alone, the variations in which determine the more
speedy or more tardy disclosure of the included insect. The eggs
of such species as have several broods in the year, as the nettle
butterfly (_Vanessa Urticæ_) when laid in summer are hatched in a
few days; but if not laid till the close of autumn, they remain
dormant through the winter, and are only hatched at the return of
spring. That this difference is to be attributed to the influence
of heat has been often proved by experiment: the autumnal eggs if
brought into a warm room may be hatched as soon as those laid in
the height of summer. Silk-worms' eggs naturally are not hatched
till they have been laid six weeks, but in countries where they are
reared, the women effect their exclusion in a much shorter period
by carrying them in their bosoms: yet to retard their hatching with
particular views is in many circumstances impossible. When the heat
of the atmosphere has reached a certain point, the hatching cannot
be retarded by cellars; and M. Faujas has remarked, that in June the
silk-worm's eggs would hatch in an ice-house[195].